The present invention relates to an image frequency reflection mode filter for distributed transmission lines implemented in planar form, for example in the form of a micro-strip. This filter is particularly suitable for use in a high-frequency receiver for television signals, this receiver comprising a mixer receiving on the one hand a receiving signal having the frequency f.sub.S and on the other hand a signal having the frequency f.sub.OL produced by a local oscillator. The mixer produces an intermediate frequency signal f.sub.FI which is equal to the difference between the frequencies f.sub.S and f.sub.OL. The filter functions to prevent the transmission of a parasitic signal having the frequency f.sub.p (the image frequency) which is equal to (2f.sub.OL -f.sub.S), and which is sent from the mixer to the input of the high-frequency receiver.
Because the mixer is a non-linear device it can produce a whole series; of other second order products when the signals of frequencies f.sub.S and f.sub.OL are combined. These are also parasitic signals, but their frequencies are further removed than f.sub.p from the useful frequency band incorporating f.sub.S and f.sub.OL. Filtration of these frequencies is not considered here.
A filter which must ensure a proper reflection of the signal of the parasitic frequency f.sub.p, while still transmitting in an optimum manner the signals having the receiving frequency f.sub.S and, if possible, the signals having the frequency f.sub.OL, must satisfy the following conditions:
the frequency band limited by the filter must be sufficiently wide to ensure that the reflection of the frequency f.sub.p will be considerable in the overall frequency band f.sub.p may occupy;
the losses must be as low as possible at the receiving frequency f.sub.S and, if possible also at the frequency f.sub.OL, to enable the use of a local oscillator of the lowest possible power;
the reference level of the filter with respect to the mixer must be properly defined and localised, so that the action of the filter is independent of the frequency.
None of the known filters satisfies all these conditions. Neither conventional filters for coupling to transmission line nor the so-called spurline filters, which are compared with one another in the article "Design of Microstrip spur-line band-stop filters" published in the periodical Microwaves, Optics and Acoustics, November 1977, vol. 1, no. 6, operate satisfactorily enough to satisfy the above-mentioned requirements. The conventional filters reflect incident power poorly and are very sensitive to external influences (metallic objects). In these two respects the spur-line filters have improved performance, but their reference level with respect to the mixer is not properly defined. Additionally, the power losses at the frequency of the local oscillator are not inconsiderable, but are at least b 3 dB.